1. Field of the Invention
The present invention relates generally to isomerization of wet hexanes.
2. Description of the Prior Art
Current catalytic materials used for isomerization of hexanes, such as n-hexane, wet hexanes, and other hexanes, are not very tolerant to water, oxygenates and sulfur compounds. The most active industrially employed catalytic materials, such as, for example, platinum on chlorided-alumina, require extensive hexanes, i.e., feed, pretreatment to completely remove water, sulfur, and oxygenates. Modern zeolite-based catalysts can tolerate up to about 200 ppmw sulfur and 200 ppmw water, although these are less active than most chlorided-alumina catalysts. Metal oxide-based catalytic materials are intermediate in activity and can tolerate contaminants of sulfur and water at operating levels of less than 20 ppmw. These catalysts are typically zirconia-based because of their ability to generate super acids through sulfation. Some commercially available catalysts have water tolerances of up to at least about 30 ppmw. Unfortunately, while improvements to catalytic materials and catalysts that are water tolerant have been made, no commercially available catalytic materials or catalysts operate effectively at conditions of greater than 200 ppmw water in the wet hexanes feed stream.
The hydrothermal stability of zirconia under aqueous phase reforming conditions makes zirconia an attractive material for processing other feed stocks having a high water content. While zirconia is, in and of itself, an amphoteric material, the addition of tungsten or molybdenum has been shown to generate considerable Brönsted acidity. This acidity, though transient in the absence of gas-phase hydrogen, is not generated by the adsorption of strong acids, such as the case of sulfated zirconia and chlorided alumina. This makes materials such as tungsten-zirconia likely to perform acid catalyzed conversions in the presence of water. The hydrothermal stability of tungsten-zirconia is alleged to be true, although this is more of an assertion than a proven fact. Tungsten-zirconia catalysts have been used for hydration of propylene, aqueous hydrolysis of esters, aqueous esterification, and the hydration of ethylene, though the yields are low.
In the area of biofuels, the hydrogenation of glucose to sorbitol followed by hydrotreating of sorbitol to n-hexane often can result in an n-hexane stream containing up to about 30 weight percent water. Upgrading this n-hexane stream without the need to eliminate water would be advantageous for process heat integration and simplicity.